I almost aways agree with Unterhuasen who gives great advice but not in this case. I think it is okay to thin the lugs. And it is great practice for when you build your own. They are overly thick on most vintage frames. I’m a framebuilder that came from the 70’s where lug thinning was considered to be a mark of excellence. The reason most frames have bulky lugs is because it was too much time and effort to file them down. All my custom frames I’ve built with lugs have been worked over a lot both before and after brazing. I work on the edges before assembly and do almost all the thinning after they are brazed. I do that for several reasons but that is beyond the scope of this subject thread.
My recommendation is to start on the sides to establish your desired lug thickness. Your goal is to have even thickness all the way around. It is much harder to remove metal there because your file strokes are limited. You have to be careful to not let the file slide off of the lug onto the tube which both rounds the edge (a mark of poor craftsmanship) and scars the tube. Holding the file at the right angle helps keep some part of it on the lug so it doesn’t entirely slip off. Most pros that thin lugs leave a thickness of 1.2 to 1.0 mm.
Basically lug filing is a 3 step process. 1st remove bulk with a course cut file, 2nd remove file marks with a fine cut file and finally polish out with 80 grit emery cloth. I use my finger as backing while others use a file or some other kind of block. Be careful with going over the lug tips with either a file or emery (particularly if using a shoe shine motion) because they can be taken down too far in the blink of an eye.